Embodiments presented herein relate generally to magnetic resonance imaging (MRI), and more particularly to mapping magnetic fields produced in MRI.
MRI is widely used for imaging of the human body for diagnostics and medical tests. MRI functions on the principle of nuclear magnetic resonance (NMR) of atoms of certain elements. One feature of NMR that enables imaging is that the resonance frequency of the nuclei of particular atoms is directly proportional to the strength of the applied magnetic field. An MRI system uses a powerful magnetic field (also known as B0 field) to align the magnetization of some atoms in the body, and radio frequency fields (also known as B1 field) to systematically alter the alignment of this magnetization. To produce high resolution images, the magnetic fields of the MRI system should be uniform, and free from distortion. Typically, MRI systems also comprise passive assemblies such as shaped metallic implements, or active assemblies such as electromagnets to homogenize and correct distortions in the magnetic field. Such homogenization is better known as shimming.
Specifically for in vivo imaging a highly homogenous magnetic field is required. Achieving such homogeneity introduces additional time consuming shimming iterations. Such additional time requirement is undesirable for human imaging operations. One known technique—“Fast Automatic Shimming Technique by Mapping Along Projections” or FASTMAP maps the B0 magnetic field along a small number of linear projections rather than imaging the entire volume of interest. FASTMAP allows for accurate B0 field shimming without iterations, and thus produces the desired B0 field homogeneity in short time duration.
Like the B0 field, in vivo imaging also requires a highly homogenous B1 field generated by the RF coils. Active shimming techniques for homogenizing the B1 field are known in the field as B1 mapping and are typically combined with spin-echo or gradient echo encoding. However, known techniques require a long time for accurate mapping of the B1 field. Some proprietary techniques for high speed B1 field mapping achieve short shimming time at the expense of resolution, and noise. Further, known B1 field mapping techniques image the complete volume of interest for accurate shimming.
Therefore, methods and systems are required for B1 field mapping, that overcome these and other problems associated with the known B1 field mapping techniques.